This invention relates to novel alkyd resins which contain polyfunctional hydroxyl compounds produced from epoxides of fatty acid derivatives or from epoxides of fatty alcohols. More particularly, the invention relates to improved alkyd resins for use in water lacquers, particularly stoving water lacquers.
Alkyd resins are polyesters of oils or fatty acids, polyfunctional carboxylic acids and polyfunctional hydroxy compounds. Although numerous compounds may be used for the production of alkyd resins, for economic reasons, the number of compounds actually used is very limited. In order therefore to satisfy increased technical requirements in special fields, there is a need for new resins based on new commercially obtainable starting materials. This is particularly applicable to the alkyd resins used in stoving water lacquers. Due to the shortage and high cost of petrochemical raw materials, natural oleochemical products are attracting particular interest.
In recent years, numerous efforts have been made to provide stoving lacquers which, on stoving, give off hardly any volatile atmosphere-polluting substances. For example, it has frequently been proposed to use as binders carboxyl group-containing alkyd resins which can be dissolved in aqueous solvent mixtures after neutralization with amines. Alkyd resins suitable for this purpose generally have acid numbers of from 40 to 80 mg of KOH/g. The use of alkyd resin dispersions which have acid numbers of from &lt;30 mg of KOH/g or even &lt;10 KOH/g, but which contain emulsifiers, is also known.
Finally, alkyd resin microemulsions are known which have acid numbers of from 5 to 35, but which contain no volatile solvents and only emulsifiers of the type which lose their hydrophilicity on stoving. Thus, U.S. Pat. No. 4,271,051 describes alkyd resins containing numerous 2,3-dihydroxy propyl groups through the reaction of free carboxyl groups with glycidol. By selecting suitable amines, emulsifiers and crosslinking agents (generally melamine resins), it is possible to produce from these alkyd resins optically clear aqueous preparations (microemulsions) which on stoving give off hardly any harmful constituents to the atmosphere.
Although these alkyd resins are largely satisfactory both in terms of processibility into water lacquers and in terms of environmental considerations, there is still a need to improve the technical properties of lacquer films obtainable therefrom. Thus, the resistance to water (as determined by the "sweatbox" test) and also the flexibility of the lacquer films, despite their considerable hardness, are in need of improvement without however producing any adverse effects upon their other favorable technical properties.
It is known from U.S. Pat. No. 4,039,495 that trimethylol nitromethane may be used as an additive for increasing the flexibility of alkyd resin lacquers. Unfortunately, this highly hydrophilic additive reduces the resistance of the lacquer films to water very considerably. Attempts to obtain flexible lacquers by increasing the fatty acid content of the alkyd resins were also unsuccessful because resins of this type could not be made up into water lacquers without the use of solvents.